Universal fluctuating regime in triangular chromate antiferromagnets

Abstract

We report x-ray diffraction, magnetic susceptibility, heat capacity, $^{1}\mathrm{H}$ nuclear magnetic resonance (NMR), and muon spin relaxation $(\ensuremath{\mu}\mathrm{SR})$ measurements, as well as density-functional band-structure calculations for the frustrated $S=3/2$ triangular lattice Heisenberg antiferromagnet (TLHAF) $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{HCrO}}_{2}$ (trigonal, space group: $R\overline{3}m$). This compound undergoes a clear magnetic transition at ${T}_{\mathrm{N}}\ensuremath{\simeq}22.5$ K, as seen from the drop in the muon paramagnetic fraction and concurrent anomalies in the magnetic susceptibility and specific heat capacity. Local probes (NMR and $\ensuremath{\mu}\mathrm{SR}$) reveal a broad regime with slow fluctuations down to $0.7\phantom{\rule{0.16em}{0ex}}{T}_{\mathrm{N}}$, this temperature corresponding to the maximum in the $\ensuremath{\mu}\mathrm{SR}$ relaxation rate and in the NMR wipe-out. From the comparison with ${\mathrm{NaCrO}}_{2}$ and $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{KCrO}}_{2}$, the fluctuating regime and slow dynamics below ${T}_{\mathrm{N}}$ appear to be hallmarks of the TLHAF with $ABC$ stacking. We discuss the role of interlayer frustration, which may have impacted recent spin-liquid candidates with triangular geometry.